The field of the present invention relates to optical apparatus. In particular, suppression of stray light propagation in a substrate is disclosed herein.
Suppression of stray light propagation in a substrate may be necessary or desirable in a variety of optical apparatus. For example, imperfect optical coupling (end-coupling or transverse-coupling) between optical waveguides, optical fibers, or optical devices such as lasers and modulators, may often result in substantial amounts of light “dumped” into a substrate on which these objects are formed or mounted. Many optical telecommunications apparatus are constructed or fabricated on high-index semiconductor substrates, which are often of high optical quality and transparent at the relevant operative wavelengths (typically between about 1200 nm and about 1700 nm; other wavelength ranges may be employed). Stray light, if allowed to freely propagate within the substrate and to reflect from the substrate surfaces, may interfere with the proper operation of the optical apparatus. Similar circumstances may arise with other substrate materials or in other wavelength ranges. Examples of circumstances wherein this might be the case include: i) optical apparatus that include one or more lasers or other light sources, which act as sources of stray light, and ii) optical apparatus that include at least one photodetector, wherein stray light incident on the photodetector may interfere with detection of an intended optical signal (so-called optical crosstalk). Various optical apparatus are disclosed herein that include an absorbing layer and a reflection-suppressing layer on the substrate for suppressing propagation of stray light in the substrate.